RE: panic! (the `impossible' happened) :-O

[Simon Peyton-Jones]

| panic! (the `impossible' happened):
|  cgEvalAlts: dodgy case of unboxed tuple type

It's a bug all right; but it's only a bug in the error reporting.
This data type declaration should be rejected, and isn't:

        data Protocol t =>  H t = H (# Int#, t #)

Unboxed tuples aren't really first class types, I'm afraid.  But
Your program is simpler without them.  Just write

        data Protocol t =>  H t = H Int# t

Simon